Buggy code:
public class Interpreter { private Map < String , Double > variables = new HashMap < String , Double > ( ) ; public Interpreter ( ) { } public int interpret ( String string ) { } private int interpretString ( String string ) { ArrayList < String > segments = new ArrayList < String > ( ) ; for ( String split_string : string . split ( "\\s+" ) ) { segments . add ( split_string ) ; } System . out . println ( ( "IN:<seq2seq4repair_space>" + segments ) ) ; int arguments = 0 ; if ( segments . get ( 0 ) . equals ( "ADD" ) ) { arguments = 3 ; if ( ( segments . size ( ) ) == ( arguments + 1 ) ) { if ( variables . containsKey ( segments . get ( 1 ) ) ) { if ( variables . containsKey ( segments . get ( 2 ) ) ) { if ( variables . containsKey ( segments . get ( 3 ) ) ) { <START_BUG> variables . put ( segments . get ( 3 ) , ( ( Double . parseDouble ( segments . get ( 1 ) ) ) + ( Double . parseDouble ( segments . get ( 2 ) ) ) ) ) ; <END_BUG> this . printOutput ( ( ( ( ( ( "ADD<seq2seq4repair_space>" + ( segments . get ( 1 ) ) ) + "<seq2seq4repair_space>by<seq2seq4repair_space>" ) + ( segments . get ( 2 ) ) ) + "<seq2seq4repair_space>and<seq2seq4repair_space>set<seq2seq4repair_space>to<seq2seq4repair_space>" ) + ( segments . get ( 3 ) ) ) ) ; } else { this . printErrorAndExit ( ( ( "name<seq2seq4repair_space>" + ( segments . get ( 3 ) ) ) + "<seq2seq4repair_space>is<seq2seq4repair_space>not<seq2seq4repair_space>defined" ) ) ; } } else { this . printErrorAndExit ( ( ( "name<seq2seq4repair_space>" + ( segments . get ( 2 ) ) ) + "<seq2seq4repair_space>is<seq2seq4repair_space>not<seq2seq4repair_space>defined" ) ) ; } } else { this . printErrorAndExit ( ( ( "name<seq2seq4repair_space>" + ( segments . get ( 1 ) ) ) + "<seq2seq4repair_space>is<seq2seq4repair_space>not<seq2seq4repair_space>defined" ) ) ; } } else { this . printErrorAndExit ( ( ( ( ( "ADD<seq2seq4repair_space>takes<seq2seq4repair_space>exactly<seq2seq4repair_space>" + arguments ) + "<seq2seq4repair_space>arguments<seq2seq4repair_space>(" ) + ( ( segments . size ( ) ) - 1 ) ) + "<seq2seq4repair_space>given)" ) ) ; } } else if ( segments . get ( 0 ) . equals ( "ASSIGN" ) ) { arguments = 2 ; if ( ( segments . size ( ) ) == ( arguments + 1 ) ) { if ( variables . containsKey ( segments . get ( 1 ) ) ) { variables . put ( segments . get ( 1 ) , Double . parseDouble ( segments . get ( 2 ) ) ) ; this . printOutput ( ( ( ( "ASSIGN<seq2seq4repair_space>" + ( segments . get ( 2 ) ) ) + "<seq2seq4repair_space>to<seq2seq4repair_space>" ) + ( segments . get ( 1 ) ) ) ) ; } else { this . printErrorAndExit ( ( ( "name<seq2seq4repair_space>" + ( segments . get ( 1 ) ) ) + "<seq2seq4repair_space>is<seq2seq4repair_space>not<seq2seq4repair_space>defined" ) ) ; } } else { this . printErrorAndExit ( ( ( ( ( "ASSIGN<seq2seq4repair_space>takes<seq2seq4repair_space>exactly<seq2seq4repair_space>" + arguments ) + "<seq2seq4repair_space>arguments<seq2seq4repair_space>(" ) + ( ( segments . size ( ) ) - 1 ) ) + "<seq2seq4repair_space>given)" ) ) ; } } else if ( segments . get ( 0 ) . equals ( "CMT" ) ) { } else if ( segments . get ( 0 ) . equals ( "DIV" ) ) { arguments = 3 ; if ( ( segments . size ( ) ) == ( arguments + 1 ) ) { if ( variables . containsKey ( segments . get ( 1 ) ) ) { if ( variables . containsKey ( segments . get ( 2 ) ) ) { if ( variables . containsKey ( segments . get ( 3 ) ) ) { variables . put ( segments . get ( 3 ) , ( ( Double . parseDouble ( segments . get ( 1 ) ) ) // ( Double . parseDouble ( segments . get ( 2 ) ) ) ) ) ; this . printOutput ( ( ( ( ( ( "DIV<seq2seq4repair_space>" + ( segments . get ( 1 ) ) ) + "<seq2seq4repair_space>by<seq2seq4repair_space>" ) + ( segments . get ( 2 ) ) ) + "<seq2seq4repair_space>and<seq2seq4repair_space>set<seq2seq4repair_space>to<seq2seq4repair_space>" ) + ( segments . get ( 3 ) ) ) ) ; } else { this . printErrorAndExit ( ( ( "name<seq2seq4repair_space>" + ( segments . get ( 3 ) ) ) + "<seq2seq4repair_space>is<seq2seq4repair_space>not<seq2seq4repair_space>defined" ) ) ; } } else { this . printErrorAndExit ( ( ( "name<seq2seq4repair_space>" + ( segments . get ( 2 ) ) ) + "<seq2seq4repair_space>is<seq2seq4repair_space>not<seq2seq4repair_space>defined" ) ) ; } } else { this . printErrorAndExit ( ( ( "name<seq2seq4repair_space>" + ( segments . get ( 1 ) ) ) + "<seq2seq4repair_space>is<seq2seq4repair_space>not<seq2seq4repair_space>defined" ) ) ; } } else { this . printErrorAndExit ( ( ( ( ( "DIV<seq2seq4repair_space>takes<seq2seq4repair_space>exactly<seq2seq4repair_space>" + arguments ) + "<seq2seq4repair_space>arguments<seq2seq4repair_space>(" ) + ( ( segments . size ( ) ) - 1 ) ) + "<seq2seq4repair_space>given)" ) ) ; } } else if ( segments . get ( 0 ) . equals ( "END" ) ) { } else if ( segments . get ( 0 ) . equals ( "ENTER" ) ) { } else if ( segments . get ( 0 ) . equals ( "INC" ) ) { } else if ( segments . get ( 0 ) . equals ( "MUL" ) ) { } else if ( segments . get ( 0 ) . equals ( "PRINT" ) ) { arguments = 1 ;

Fix:
variables . put ( segments . get ( 3 ) , ( ( variables . get ( segments . get ( 1 ) ) ) + ( variables . get ( segments . get ( 2 ) ) ) ) ) ;